U.S. patent application number 12/486938 was filed with the patent office on 2010-06-03 for method of preparing spherical shape positive active material for lithium secondary battery.
This patent application is currently assigned to HYUNDAI MOTOR COMPANY. Invention is credited to Dong Gun Kim, Kyung Su Kim, Sa Heum Kim, Young Jun Kim, Jun Ho Song.
Application Number | 20100136433 12/486938 |
Document ID | / |
Family ID | 42223134 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100136433 |
Kind Code |
A1 |
Kim; Kyung Su ; et
al. |
June 3, 2010 |
METHOD OF PREPARING SPHERICAL SHAPE POSITIVE ACTIVE MATERIAL FOR
LITHIUM SECONDARY BATTERY
Abstract
The present invention relates to a process of preparing a
spherically-shaped positive active material for a lithium secondary
battery, comprising: (a) uniformly dissolving a raw material
mixture comprising a lithium-based compound, a transition metal,
phosphate-based compound and a carbon source in deionized water;
(b) preparing a high density spherically-shaped precursor by
rapidly freezing the mixed solution in a freeze granulator and
sublimating the frozen mixed solution; and (c) thermally treating
the high density spherically-shaped precursor. The thus-prepared
spherically-shaped positive active material is superior in
crystallinity and electric conductivity, thus being useful in the
manufacture of an electrode for a lithium secondary battery.
Inventors: |
Kim; Kyung Su; (Seoul,
KR) ; Kim; Sa Heum; (Gyeonggi-do, KR) ; Kim;
Dong Gun; (Seoul, KR) ; Kim; Young Jun;
(Gyeonggi-do, KR) ; Song; Jun Ho; (Gyeonggi-do,
KR) |
Correspondence
Address: |
EDWARDS ANGELL PALMER & DODGE LLP
P.O. BOX 55874
BOSTON
MA
02205
US
|
Assignee: |
HYUNDAI MOTOR COMPANY
Seoul
KR
KOREA ELECTRONICS TECHNOLOGY INSTITUTE
|
Family ID: |
42223134 |
Appl. No.: |
12/486938 |
Filed: |
June 18, 2009 |
Current U.S.
Class: |
429/231.95 ;
252/519.3 |
Current CPC
Class: |
H01M 10/052 20130101;
Y02E 60/10 20130101; H01M 4/623 20130101; H01M 4/621 20130101; H01M
4/625 20130101; H01M 4/5825 20130101 |
Class at
Publication: |
429/231.95 ;
252/519.3 |
International
Class: |
H01M 4/58 20060101
H01M004/58; H01B 1/12 20060101 H01B001/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 28, 2008 |
KR |
10-2008-0120056 |
Claims
1. A process of preparing a spherically-shaped positive active
material for a lithium secondary battery, the process comprising:
(a) preparing a raw material mixture comprising a lithium-based
compound, a transition metal, phosphate-based compound and a carbon
source; (b) preparing a mixed solution by uniformly dissolving the
raw material mixture in deionized water; (c) rapidly freezing the
mixed solution in a freeze granulator; (d) preparing a high density
spherically-shaped precursor by sublimating the frozen mixed
solution; and (e) thermally treating the high density
spherically-shaped precursor.
2. The process of claim 1, wherein the mixed solution comprises
8-12 wt % of the lithium-based compound, 40-50 wt % of the
transition metal, 20-25 wt % of the phosphate-based compound and
0.1-30 wt % of the carbon source.
3. The process of daim 2, wherein the lithium-based compound is
selected from the group consisting of LiPO.sub.4, Li.sub.2CO.sub.3,
LiOH and acetate-lithium; the transition metal is an
iron-containing compound selected from the group consisting of
FeSO.sub.4.7H.sub.2O, FeC.sub.2O.sub.4.2H.sub.2O, iron oxalate and
iron acetate; the phosphate-based compound is selected from the
group consisting of phosphoric acid, ammonium phosphate and
(NH.sub.4).sub.2HPO.sub.4; and the carbon source is citric
acid.
4. The process of claim 1, wherein the molar ratio of the
lithium-based compound and the transition metal is 0.95-1.10:1.
5. The process of claim 2, wherein the thermal treatment is
conducted in a mixed gas comprising 3-7% of hydrogen and 93-97% of
argon at 500-800.degree. C. for 1-10 hours.
6. The process of daim 2, wherein the positive active material has
a partide size of 5-20 .mu.m.
7. The process of claim 1, wherein the positive active material is
LiFePO.sub.4/C.
8. The process of claim 7, wherein the LiFePO.sub.4/C has a
lithium/iron molar ratio of 0.98-1.02:1, and a phosphorus/iron
molar ratio of 0.98-1.02:1.
9. A spherically-shaped positive electrode for a lithium secondary
battery prepared according to a process comprising: (a) preparing
35-42% of a solid matter by dissolving in N-methyl pyrrolidone
(NMP) a mixture comprising (i) 85-90 wt % of the spherically-shaped
positive active material prepared according to claim 1, (ii) 1-10
wt % of a conductive material and (iii) 1-10 wt % of a binder; (b)
coating the solid matter on an aluminum-foil; (c) drying the coated
solid matter at 110-120.degree. C.; and (d) roll-pressing the dried
solid matter at 2 g/cc.
10. The spherically-shaped positive electrode for a lithium
secondary battery of claim 9, wherein the conductive material is
super-P+ carbon nanotube (vapor growth carbon fiber) and the binder
is polyvinylidene fluoride (PVDF).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims under 35 U.S.C. .sctn.119(a) the
benefit of Korean Patent Application No. 10-2008-0120056 filed Nov.
28, 2008, the entire contents of which are incorporated herein by
reference.
BACKGROUND
[0002] (a) Technical Field
[0003] The present invention relates to a process of preparing a
spherically-shaped positive active material for a lithium secondary
battery, and particularly to a process of preparing the same, which
comprises (a) uniformly dissolving a raw material mixture
comprising a lithium-based compound, a transition metal,
phosphate-based compound and a carbon source in deionized water,
(b) preparing a high density spherically-shaped precursor by
rapidly freezing the mixed solution in a freeze granulator and
sublimating the frozen mixed solution, and (c) thermally treating
the high density spherically-shaped precursor.
[0004] (b) Background Art
[0005] With the rapidly increasing demand for portables such as
laptop computers, camcorders, mobile phones and small-sized
recorders and the recent trend of reduction in size of these
portables, a lithium secondary battery used as an energy source for
these portables have been developed so as to increase energy
density and extend operable time. The most important part in a
lithium secondary battery is the materials for negative and
positive electrodes. In particular, it is essential that the
material for a lithium secondary battery positive electrode meet
the requirements of relatively high discharge capacity, low price,
superior cycleability for increasing the operable time and thermal
and structural stabilities with no explosion risk.
[0006] Transition metal oxides having a layered or spinel structure
are widely used as positive active material for a lithium secondary
battery. Recently, lithium transition metal phosphate positive
active material with a superior stability has been widely studied.
In particular, attention has been drawn to LiFePO.sub.4 of olivine
structure because of its high theoretical capacity (170 mAh/g) and
superior high-temperature stability and a low price due to the use
of Fe, despite its relatively low voltage (3.4 V lower than that of
lithium).
[0007] LiFePO.sub.4 shows a relatively high discharge capacity, and
is low priced because comparatively cheap Fe is used instead of Co.
LiFePO.sub.4 is also eco-friendly because no heavy metal is
contained. Moreover, LiFePO.sub.4 is chemically and structurally
stable and shows a relatively long usable time. In particular,
LiFePO.sub.4 shows a remarkable thermal stability, thus being
appropriate for the positive electrode material of an automotive
lithium secondary battery. However, a change in the oxidation from
Fe.sup.2+ to Fe.sup.3+ during the manufacture of LiFePO.sub.4
should be avoided. Pure LiFePO.sub.4 is significantly low in
Li-expansion coefficient (10.sup.-14 cm.sup.2/s) and electric
conductivity (10.sup.-8-10.sup.-9 s/cm), and LiFePO.sub.4 is thus
much lower than LiCoO.sub.2 in a rate capability.
[0008] In a conventional process of preparing LiFePO.sub.4,
processing the mixture of solid-phase Li.sub.2CO.sub.3,
NH.sub.4H.sub.2PO.sub.4 and FeC.sub.2O.sub.4 is conducted at
800.degree. C. under an argon atmosphere. LiFePO.sub.4 is
relatively low in electric conductivity, thus requiring an
additional step of mixing LiFePO.sub.4 with carbon for increasing
the conductivity of electrodes.
SUMMARY OF THE DISCLOSURE
[0009] One aspect of the present invention provides a process of
preparing a spherically-shaped positive active material for a
lithium secondary battery, which comprises (a) uniformly dissolving
in deionized water a raw material mixture comprising a
lithium-based compound, a transition metal, phosphate-based
compound and a carbon source, (b) preparing a high density
spherically-shaped precursor by rapidly freezing the mixed solution
in a freeze granulator and sublimating the frozen mixed solution,
and (c) thermally treating the high density spherically-shaped
precursor.
[0010] Another aspect of the present invention provides a process
of preparing a spherically-shaped positive active material for a
lithium secondary battery, which comprises (a) preparing a raw
material mixture comprising a lithium-based compound, a transition
metal, phosphate-based compound and a carbon source; (b) preparing
a mixed solution by uniformly dissolving the raw material mixture
in deionized water; (c) rapidly freezing the mixed solution in a
freeze granulator; (d) preparing a high density spherically-shaped
precursor by sublimating the frozen mixed solution; and (e)
thermally treating the high density spherically-shaped
precursor.
[0011] The above and other features of the invention are discussed
infra.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The above and other features of the present invention will
now be described in detail with reference to certain exemplary
embodiments thereof illustrated the accompanying drawings which are
given hereinbelow by way of illustration only, and thus are not
limitative of the present invention, and wherein:
[0013] FIG. 1 shows the energy density of positive active material
(LiFePO.sub.4/C composite material) prepared by a process according
to the present invention;
[0014] FIG. 2 shows the energy density of positive active material
prepared by a conventional method;
[0015] FIG. 3 schematically shows a freeze granulation
procedure;
[0016] FIG. 4 shows the particle shape of precursors prepared by a
process according to the present invention;
[0017] FIG. 5 is SEM (scanning electron micrograph) images showing
the particle shape of synthesized lithium phosphate; and
[0018] FIG. 6 is XRD data of synthesized lithium phosphate positive
active material.
[0019] It should be understood that the appended drawings are not
necessarily to scale, presenting a somewhat simplified
representation of various preferred features illustrative of the
basic principles of the invention. The specific design features of
the present invention as disclosed herein, including, for example,
specific dimensions, orientations, locations, and shapes will be
determined in part by the particular intended application and use
environment.
[0020] In the figures, reference numbers refer to the same or
equivalent parts of the present invention throughout the several
figures of the drawing.
DETAILED DESCRIPTION
[0021] Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the drawings attached hereinafter, wherein like
reference numerals refer to like elements throughout. The
embodiments are described below so as to explain the present
invention by referring to the figures.
[0022] A raw material mixture comprising a lithium-based compound,
a transition metal, a phosphate-based compound and a carbon source
is prepared. The raw material comprises 8-12 wt % of the
lithium-based compound, 40-50 wt % of the transition metal, 20-25
wt % of the phosphate-based compound and 13-32 wt % of the carbon
source.
[0023] Non-limiting examples of the lithium-based compound include
LiPO.sub.4, Li.sub.2CO.sub.3, LiOH and acetate-lithium
(Li-acetate). Non-limiting examples of the transition metal include
an iron-containing compound selected from the group consisting of
FeSO.sub.4.7H.sub.2O, FeC.sub.2O.sub.4.2H.sub.2O, iron
oxalate(Fe-oxalate) and iron acetate (Fe-acetate). Non-limiting
examples of the phosphate-based compound include phosphoric acid
and ammonium phosphate ((NH.sub.4).sub.2HPO.sub.4). A preferable
example of the carbon source is citric acid with superior electric
conductivity.
[0024] A preferable molar ratio of a lithium-based compound to a
transition metal is in the range of 0.95-1.10:1. If the molar ratio
is less than 0.95:1, the production of impurities such as
Fe.sub.2O.sub.3 and Fe.sub.2P can increase. If the molar ratio is
more than 1.10:1, the production of impurities such as
Li.sub.2CO.sub.3 and LiOH can increase.
[0025] The raw material mixture is uniformly dissolved in deionized
water (DI water) to provide a mixed solution.
[0026] The mixed solution is rapidly frozen in a freeze granulator
to provide a spherically-shaped precursor.
[0027] The present invention adopts a freezing method and differs
from the conventional process using a solid-phase method. Unlike
the conventional method, the present method can uniformly mix raw
materials. The present invention also efficiently overcomes the
problem of a relatively low energy density per unit volume due to
small secondary particles in a conventional process of preparing an
olivine positive electrode material. Energy density per unit volume
can be controlled by adjusting the size of a spherically-shaped
precursor, thereby improving reversible capacity of a battery.
[0028] The frozen spherically-shaped precursor is sublimated to
provide high density spherically-shaped precursor. The
spherically-shaped precursor is sublimated in a drier at
-10-0.degree. C. and 10.sup.-5-10.sup.-1 Pa for a period of time
between 6 minutes and 6 hours. Water can be removed and high
density spherically-shaped precursor can be prepared by this
sublimation step, thereby enabling to control the amount of
remaining carbon.
[0029] Then, the thus-obtained high density spherically-shaped
precursor is thermally treated. The thermal treatment is conducted
under a reductive atmosphere in a gas mixture containing 3-7% of
hydrogen and 93-97% of argon at 500-800.degree. C. for 1-10 hours.
Thus prepared spherically-shaped positive active material for
lithium secondary battery has a particle size of 5-20 .mu.m. If the
particle size is less than 5 .mu.m, the degree of granulation
decreases drastically. When the particle size in more than 20
.mu.m, the time necessary for lithium ions to migrate into
particles can be extended, thereby deteriorating power output
characteristics of the battery.
[0030] The final spherically-shaped positive active material for a
lithium secondary battery prepared is LiFePO.sub.4/C. The molar
ratios of Li/Fe and P/Fe are 0.98-1.06:1 and 0.98-1.02:1,
respectively.
[0031] Hereunder is provided a detailed description of a process of
preparing a spherically-shaped positive electrode for a lithium
battery by using the aforementioned spherically-shaped positive
active material.
[0032] Solid matter (35-42%) is prepared by dissolving in N-methyl
pyrrolidone (NMP) a mixture comprising (i) 85-90 wt % of the
aforementioned spherically-shaped positive active material for
lithium secondary battery, (ii) 1-10 wt % of a conductive material
and (iii) 1-10 wt % of a binder. The solid matter is coated on an
aluminum-foil, and dried at 110-120.degree. C., followed by the
roll-pressing of the dried solid matter at 2 g/cc. Although any
known materials can be used as the conductive material and the
binder, super-P+ carbon nanotube (vapor growth carbon fiber) and
polyvinylidene fluoride (PVDF) are preferred for the conductive
material and the binder, respectively.
EXAMPLES
[0033] The following examples illustrate the invention and are not
intended to limit the same.
[0034] Equivalent amount of a lithium-based compound
Li.sub.3PO.sub.4, a transition metal FeC.sub.2O.sub.4.2H.sub.2O and
phosphate-based compounds Li.sub.3PO.sub.4 and
(NH.sub.4).sub.2HPO.sub.4 were dissolved in deionized water to
provide a raw material mixture. 20 wt % of citric acid was added as
a carbon source. The mixed solution was rapidly frozen in a freeze
granulator (FIG. 3) and sublimated, followed by the removal of
water at -5.degree. C. and 10.sup.-3 Pa for 3 hours. The dried
mixed solution was thermally treated under a 5% argon atmosphere at
500.degree. C. for 5 hours to synthesize LiFePO.sub.4/C.
[0035] The heat capacity versus voltage of the LiFePO.sub.4/C was
shown in FIG. 1. LiFePO.sub.4/C prepared herein shows a superior
energy density (160 mAh/g), which amounts to one at least 30%
improved compared to those of the conventionally prepared
LiFePO.sub.4/C (FIG. 2, 120 mAh/g).
[0036] A process of preparing a spherically-shaped positive active
material for a lithium secondary battery of the present invention
can overcome the problem of environmental pollution caused by the
use of organic solvent in the mechanical pulverization and the
sol-gel method. A process herein can also overcome the difficulty
in preparing electrodes using nano-sized positive active material.
The addition of carbon microparticles in the present invention
enables the manufacture of active material with superior electric
conductivity. Moreover, the raw material can be uniformly mixed in
the present invention, thereby remarkably increasing the
crystallinity. The spherical shape of particles is advantageous in
the production of material useful for the preparation of
electrodes.
[0037] The invention has been described in detail with reference to
preferred embodiments thereof. However, it will be appreciated by
those skilled in the art that changes may be made in these
embodiments without departing from the principles and spirit of the
invention, the scope of which is defined in the appended claims and
their equivalents.
* * * * *